Watchdog: omap_wdt: add fine grain runtime-pm

The omap_wdt should only be in full active state when the
registers are being accessed. Otherwise the device can be
on lower power mode.

This patch is based on a patch created by Kalle Jokiniemi:
https://patchwork.kernel.org/patch/618231/
which is itself based on a patch created by Atal
Shargorodsky: http://lkml.org/lkml/2009/3/10/266

.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@nokia.com>
Tested-by: Kalle Jokiniemi <kalle.jokiniemi@nokia.com>
Cc: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Kevin Hilman <khilman@ti.com>